1. Field of the Invention
The present invention relates to a variable-passband variable-phase digital filter. Potential applications include filtering of audio-frequency signals and television signals as well as the construction of many electronic devices such as frequency synthesizers, for example.
2. Description of the Prior Art
Variable-passband variable-phase digital filters also find an application in the audiovisual field for removing defects which appear at a time of geometrical transformations carried out with digital image memories, in particular in the event of anamorphic effects and compressions which produce moire effects and flicker effects on the transformed image.
Digital filters at present in use generally have transversal or recursive filter structures constituted by arrays of multiplier circuits, the variation in passband being obtained by weighting of the multiplier coefficients of the digital signal samples stored within registers of the filter.
Typically, n sets of coefficients are usually employed for the purpose of obtaining n different passbands.
Concurrently with the passband variation, digital filters corresponding to the mode of utilization just mentioned sometimes have structures which also permit interpolation of m intermediate values between two successive samples of the signal, with the result that m.times.n sets of weighting coefficients are usually applied to these filters. In the known examples of construction, n is of the order of 16 and m is of the order of 4.
The structures of digital filters constructed by means of multiplier circuits have the main disadvantage of being costly.
It is possible to overcome this disadvantage by constructing variable-passband and variable-phase filters in accordance with a table look-up technique. In this technique, results which are equivalent to those mentioned in the foregoing can be obtained by constructing a table of numerical values stored within a memory which provides the filtered values of the signal as a function of the digital samples of the signal to be filtered which are applied to the memory-addressing inputs of the memory. Depending on the applications which are contemplated, it is equally possible to store said table either in a read-only memory (ROM) or in a random-access memory (RAM). The application of this structure to the problem of filtering of digital-signal samples does not present any difficulty. It is, in fact, only necessary to apply for example to the addressing inputs of the memory N bits for selecting 2.sup.N filters, M bits for selecting 2.sup.M phases and p.times.8 bits for applying the p "signal" inputs. There is thus obtained, for example, on the outputs of the memory an output signal coded on 8 bits corresponding to each address combination applied to the addressing inputs of the memory. Unfortunately, this structure cannot be realized directly when the numbers of passbands, phases and "signal" inputs become substantial. For example, in order to construct a filter having sixteen passbands, four phases and three inputs, this technique in fact entails the need for a table composed of 1000 mega-octets with thirty address lines, which is clearly neither technically nor economically feasible.